This invention relates to the deposition of thin films, and, more particularly, to deposition processes that use ionized clusters.
The deposition of thin films upon substrates is an important manufacturing and research tool in a variety of fields. For example, microelectronic devices are prepared by depositing successive film layers onto a substrate to obtain specific electronic properties of the composite. Photosensitive devices such as vidicons and solar cells are manufactured by depositing films of photosensitive materials onto substrates. Optical properties of lenses are improved by depositing films onto their surfaces. These examples are, of course, only illustrative of the thousands of applications of thin film deposition techniques.
In the highly controlled approach to thin-film deposition that is characteristic of applications where a high quality film is required, the film is built up by successive deposition of monolayers of the film, each layer being one atom thick. The mechanics of the deposition process can best be considered in atomistic terms. Generally, in such a process the surface of the substrate must be carefully cleaned, since minor contaminant masses or even contaminant atoms can significantly impede the deposition of the required highly perfect film. The material of the film is then deposited by one of many techniques developed for various applications, such as vapor deposition, electron beam evaporation, sputtering, or chemical vapor depostion, to name a few.
In another technique for depositing thin films, ionized clusters of atoms are formed in a cluster source. These clusters usually have on the order of about 1000 atoms. The clusters are ionized and then accelerated toward the substrate target by an electrical potential that imparts an energy to the cluster equal to the accelerating voltage times the ionization level of the cluster. Upon reaching the surface of the substrate, the clusters disintegrate at impact. Each atom fragment remaining after disintegation has an energy equal to the total energy of the cluster divided by the number of atoms in the cluster. The cluster prior to disintegration therefore has a relatively high mass and energy, while each atom remaining after disintegration has a relatively low mass and energy. The energy of the atom deposited upon the surface gives it mobility on the surface, so that it can move to kinks or holes that might be present on the surface. The deposited atom comes to rest in the imperfections, thereby removing the imperfection and increasing the perfection and density of the film. Other approaches to using clusters have been developed, and it appears that deposition using cluster beams is a promising commercial film manufacturing technique.
Presently available cluster beam techniques still have significant drawbacks, however. There is some surface damage induced by the beam as the film grows. This type of damage produces just the type of defect that the cluster beam approach is intended to avoid, and does avoid in part for the reasons just stated.
Accordingly, there is a continuing need for an improvement to the cluster beam approach that avoids the damage induced in the substrate by the beam. The improvement must be compatible with the cluster beam approach, and with apparatus for cluster beam thin film fabrication and the systems to which the technique is applied. The present invention fulfills this need, and further provides related advantages.